Shown in FIG. 18 is a block diagram illustrating a constitutional example of a receiver 1 of an infrared ray remote control. In the receiver 1, an infrared ray signal is converted into a light current signal Iin shown in FIG. 15(a), by a photodiode 2, so that current-voltage conversion of the light current signal Iin is carried out by an amplifier 203. Then, the light current signal Iin is amplified by an amplifier 204, and is inputted into a band pass filter 7. In the band pass filter 7, a carrier frequent component is detected, as indicated by a reference numeral α1 in FIG. 15(b). Further, as indicated by a reference numeral α11 in FIG. 15(c), the detection circuit 6 detects a transmission code component having a base band frequency out of the carrier frequency component. A resultant of the detection (detection output) is compared with a predetermined threshold level, which is indicated by a reference numeral α12, by an output circuit 10. The comparison judges whether or not the carrier exists (in other words, judeged are in which part of the signal the carrier is included in, and in which part it is not), so as to demodurate the code signal in digital. The code signal in digital is outputted as an output signal Dout (see FIG. 15(d)) from the output circuit 10. The output circuit 10, which is made of the detection circuit 8 and a hysteresis comparator, configures a carrier detection circuit.
Shown in FIG. 14 is a block diagram illustrating a concrete constitutional example of the receiver 1. The receiver 1 converts a transmission code signal of an infrared ray into the light current signal Iin (see FIG. 15(a)) by a externally-attached photodiode 2, so that the light current signal Iin is received by a receiving chip 3, which is so reduced in size that the chip 3 can be applied in an integrated circuit. The receiver chip 3 demodulates the output signal Dout (see FIG. 15(d)) so as to output the output signal Dout to an apparatus such as a microcomputer for controlling electronic apparatuses. The infrared signal is an ASK signal that has been demodulated in accordance with a predetermined carrier having a frequency of about 30 to 60 kHz.
In the receiving chip 3, the light current signal Iin is amplified subsequently by a first amplifier (HA) 4, a second amplifier (2nd AMP) 5, and a third amplifier (3rd AMP) 6, and then is transmitted to a band pass filter (BPF) 7. The band bass filter 7, which is arranged to be suitable for a frequency of a carrier, detects a carrier frequency component (indicated by a reference numeral α1 in FIG. 15(b)). Then, the carrier frequency component is detected in accordance with a carrier detection level Det (indicated by a reference numeral α2 in FIG. 15(b)) (which will be explained later), by a detection circuit 8 of the next stage. Further, a time in which the carrier exists is integrated (indicated by a reference numeral α11 in FIG. 15(c)) by an integration circuit 9. A resultant of the integration, which is an integral output Int, is compared with a predetermined threshold level (indicated by a reference numeral α12 in FIG. 15(c)) by a hysteresis comparator 10. As a result of the comparison, it is judged whether or not the carrier exists. A resultant of the judgement is outputted as the output signal Dout (see FIG. 15(d)) in digital (that is, as a digital output).
An output from the first amplifier 4 is sent to a low pass filter 11, which detects a direct current level from a fluorescent lamp or sunlight. The second amplifier 5 of the next stage amplifies a signal that is a resultant of subtraction of the direct current form the output of the first amplifier 4. This allows to avoid the effect caused by the fluorescent lamp or the sunlight. Moreover, an ABCC circuit 12 is provided in association with the first amplifier 4. The ABCC circuit 12 controls a direct current bias of the first amplifier 4 in accordance with an output of the low pass filter 11. Furthermore, an fo trimming circuit 13 is provided in association with the band pass filter 7. The fo trimming circuit 13 trims a zener diode (not shown) provided between terminals TRM 1 to TRM 5 (not shown), which are extended from junctions of voltage dividing resistors, thereby adjusting a center frequency fo of the band pass filter 7. Configured with the detection circuit 8 and the integration circuit 9 is a carrier detection circuit.
Shown in FIG. 16 is a diagram of an equivalent circuit of a typical conventional carrier detection circuit 20. The carrier detection circuit 20 is provided with a detection circuit 21, an integration circuit 22 and a hysteresis comparator (not shown). The detection circuit 21 and the integration circuit 22 are respectively in correspondence with the detection circuit 8 and integration circuit 9, previously discussed. The detection circuit 21 decides the carrier detection level Det in accordance with whether or not the carrier exists, by charging a current Ij in a capacitor C1 or alternatively discharging If from the capacitor C1, by using an amplifier 23 of a current outputting type. (Note that, the abbreviation “C1” may alternatively mean a capacity of the capacitor C1.) In FIG. 17, FIG. 15(b) is enlarged and illustrated. In short, the detection circuit 21 compares (a) an output Sig from the band pass filter 6, which is shown by the reference numeral α1 with (b) the carrier detection level Det indicated by the reference numeral α2. When the output Sig is greater than the carrier detection level Det, the capacitor C1 is charged with current Ij, while the current If is discharged from the capacitor C1 if the output Sig is smaller than the carrier detection level Det. The period for charging is a sum of periods of time indicated by a reference numeral ton in FIG. 17, while that for discharging is a sum of periods of time indicated by a reference numeral toff in FIG. 17. Therefore, the detection circuit 21 creates the carrier detection level Det in which the following condition is satisfied:
                                          1            C1                    ⁢                                    ∫              o              tonsum                        ⁢            Ij                          =                              1            C1                    ⁢                                    ∫              0              toffsum                        ⁢                                                  ⁢                                          If                ⁢                                                                  (                                                      tonsum                    =                                          ∑                      ton                                                        ,                                      toffsum                    =                                          ∑                      toff                                                                      )                            .                                                          Equation        ⁢                                  ⁢                  (          1          )                    
As described in FIG. 17, the times ton and toff are a charging time and a discharging time for the capacitor C1, respectively. Therefore, the carrier detection level Det, that is, charging voltage of the capacitor C1 varies the times ton and toff. For example, an increase in the carrier detection level Det causes the charging time ton to shorten, while lengthening the discharging time toff. Accordingly, the carrier detection level Det is equal to a level that satisfies the equation 1, in other words, a level in which a value obtained by integrating the charging current Ij with the charging time ton is equal to a value found by performing the integration of the discharging current If with the discharging time toff.
The integral circuit 22, provided with a current output amplifier 24 and a capacitor C2, compares the output Sig of the band pass filter 6 with the carrier detection level Det, and outputs a current to the capacitor C2 in accordance with the result of the comparison, thereby integrating the time during which the carrier exists, so as to be outputted as the integral output Int.
The carrier detection level Det is increased as steady noises, such as a noise from the fluorescent lamp, are inputted. It is necessary to properly set the carrier detection level Det, in order to reduce a malfunction due to the noises. For example, in case a transmission cord is transmitted with some dormant period in-between, it is likely that the noises are detected in the transmission code that is transmitted during such a dormant time when the carrier detection level Det is lowered during the dormant period. Because of this, the capacitor C1 must have a time constant for discharging (hereinafter, it may be denoted as a discharging time constant) to be longer than the dormant period, generally 100 msec or longer, considering a single transmission code of the infrared ray remote control is about 50 msec long. On the other hand, integration of chips has been demanded for a sake of a lower cost. Thus, it is desirable to use a capacity value high enough to attain integration of about 100 pF or less.
However, in the above conventional art, it is necessary to have a circuit having high impedance in which a very feeble current is used, in order to attain a 100 msec or more time constant for discharging, where a capacitor C1 is of 100 pF or less. For example, where the capacity value is at 100 pF, and the carrier detection level Det is varied within 100 mV or less in a period of time of 100 msec, a current I for charging and discharging is determined as follows:
                                                                        I                _                            =                                                (                                      C                    ×                    V                                    )                                /                T                                                                                        =                                                                    (                                          100                      ⁢                                              (                                                  p                          ⁢                                                                                                          ⁢                          F                                                )                                            ×                      100                      ⁢                                              (                                                  m                          ⁢                                                                                                          ⁢                          V                                                )                                                              )                                    /                  100                                ⁢                                  (                  mse                  )                                                                                                        =                              100                ⁢                                                      (                                          p                      ⁢                                                                                          ⁢                      A                                        )                                    .                                                                                        Equation        ⁢                                  ⁢                  (          2          )                    As shown by the Equation (2), the current I will be a very feeble current.
Such a feeble current is incapable of operating the circuit normally, due to effects by responses of transistors, unevenness per unit of the transistor in terms of properties, a parasitic (incidental) light current by light strayed from the fluorescent lamp, an incandescent lamp and the sun light, for example. In short, the high impedance of the circuit, which requires a faster speed for responding to the noises, resultants in inaccurate detection of peaks of the noises, so that the carrier detection level Det cannot be obtained from the peaks of the noises. Because of this, such a problem is posed that the carrier is not accurately detected. Accordingly, it is difficult for the capacitor C1 to be integrated in the receiving chip 3 and function normally, when the carrier detection Det is determined in accordance with the equation (1).